In a DC-DC converter that drives switching elements to step up or step down a voltage, if an excessive current flows through a power supply line due to a short circuit fault occurring in one of the switching elements, it is necessary to immediately detect an overcurrent state so that devices can be protected. For example, according to the technology disclosed in JP 2009-5555A, in a step-down DC-DC converter that is provided with a plurality of voltage step-down units, a current value is taken in from the high-voltage side and the low-voltage side of a switching element in each voltage step-down unit. Then, whether or not a failure has occurred is monitored by continuously determining whether or not a difference among the current values is greater than a predetermined value. Upon a failure being detected, the output is limited in order to prevent a non-faulty voltage step-down unit from being overloaded.
However, if a short circuit fault occurs in a switching element of a voltage step-down unit itself, it is not possible to prevent a short circuit state by simply using the method according to JP 2009-5555A, and a large current flows from the high-voltage side to the low-voltage side, which leads to an excessive voltage being unexpectedly applied to the low-voltage side. To solve this problem, it is necessary to additionally provide a configuration that can immediately block a short circuit path upon a short circuit fault occurring in a switching element of a voltage step-down unit itself. Furthermore, it is necessary to protect a circuit not only when a short circuit faulty occurs, but also when a power supply is connected the wrong way round, and there is demand for a configuration that can realize both short circuit protection and reverse connection protection.
An example of a DC-DC converter that can realize both short circuit protection and reverse connection protection is shown in FIG. 11. A DC-DC converter 100 in FIG. 11 is an example of a step-down DC-DC converter, and is configured to step down a DC voltage applied to an input line 102A on the primary side, by switching a MOSFET 104 on the high side and a MOSFET 106 on the low side, and to output the resulting voltage to an output line 102B on the secondary side. The DC-DC converter 100 monitors a current flowing through a power supply conductive path 102, using a current detection unit (not shown). For example, if an excessive current is generated in the power supply conductive path 102 due to a short circuit occurring in the MOSFET 104, the DC-DC converter 100 detects an overcurrent state and performs control to block switching elements 108 for protection. The switching elements 108 also serve as elements for reverse connection protection. For example, if a terminal 112 has a negative potential due to reverse connection in which the positive electrode and the negative electrode of the secondary-side power supply unit are connected the wrong way round, and a large current flows to the secondary side due to this reverse connection, control may be performed to block the switching elements 108 for protection, upon the occurrence of the large current being detected.
However, in the step-down DC-DC converter 100 shown in FIG. 11, a current flowing through the output line 102B on the secondary side (the low-voltage side) is larger than a current flowing through the input line 102A on the primary side (the high-voltage side). Therefore, as in the configuration shown in FIG. 11, if the switching elements 108 for protection are provided on paths on the secondary side (the low-voltage side) through which a large current flows, conduction loss in the switching elements 108 increases, and the amount of heat generated due to the conduction loss also increases.
The present invention has been made in view of the above-described situation, and aims to realize a DC-DC converter that is provided with a protection function for handling a reverse connection state, and a protection function for handling a predetermined abnormality other than a reverse connection state, while reducing conduction loss.